1. Field of the Invention
This invention relates to inertial sensing devices and the like and more particularly to a two-axis electrostatic rate gyroscope of the captured type.
2. Description of the Prior Art
Gyroscopes are customarily employed in inertial navigation and guidance systems to provide one or more fixed reference attitudes or positions about one or more axes, so that the attitude or rate of change of attitude of the vehicle in which the inertial system is mounted may be measured or sensed with respect to the fixed reference attitude. In a gyroscope of the "captured" type, the angular rotation of the spin axis of the rotor about the precession axis or axes in response to an input rate from the vehicle is restrained by a torquing or capture system which maintains the rotor in a centered or "null" position and the magnitude and direction of the restraining torque are sensed by force transducer means to obtain the vehicle attitude or rate of change of attitude as the case may be.
Present day inertial navigation and guidance systems impose high accuracy requirements upon the gyroscopes employed in the systems. In particular, a large dynamic range is desired. The dynamic range of a gyroscope is generally defined as the ratio of maximum measurable rate to the accuracy of that measurement. A satisfactory dynamic range for modern applications would be of the order of 10.sup.8 to 1. In addition to a large dynamic range, a suitable gyroscope for inertial systems should produce variable frequency output signals which may easily be processed into digital form for use with the digital information handling and processing techniques employed in modern system technology. A suitable gyroscope should also have a mechanically rugged, yet simple, construction which facilitates manufacture, maintenance and repair of the instrument.
One solution to the problem of providing a gyroscope having the aforementioned characteristics would involve the mounting of a gyroscope rotor and housing on a flexure joint having two degrees of rotational freedom corresponding to the two precession axes of the gyroscope and employing vibrating beam force transducers to restrain rotation of the rotor and housing about the two precession axes. Suspension and capture systems would suspend the rotor for rotation within the housing and "capture" the rotor with respect to the housing, so that the restraining force or torque exerted by the force transducers on the housing would be a measure of the input rate to the gyroscope. Since the vibrating beam type of force transducer produces an output signal having a variable frequency which is a function of the mechanical stress produced in the transducer by the force being sensed, the variable output frequency will be a function of the input rate applied to the gyroscope. Although this solution appears to be expedient, it presents difficulties of its own. The two-degree-of-freedom flexure joint on which the glyroscope rotor and housing assembly is mounted generally comprises a shaft or elongated support member having a "necked-down" section of gradually decreasing cross-sectional area which restrains the rotor and housing assembly against translatory movement along three orthogonal gyroscope axes but permits rotational movement about two of the axes. Since the vibrating beam force transducers should operate in a vacuum, the gyroscope casing is usually evacuated with the result that the heat flow between different sections of the gyroscope is largely by conduction rather than convection or radiation. Since the cross-sectional area of the flexure joint is small, the heat flowing by conduction through that section is limited, so that large thermal gradients and thermal distortions can be produced in the gyroscope to cause substantial errors in the output of the vibrating beam force transducers. A second difficulty with the proposed solution involves the high sensitivity of this apparatus to effects of vibration, shock and acceleration. Since the gyroscope rotor and housing assembly is restrained from rotation about the precession axes by the vibrating beam force transducers, it is important that the rotor should have as small a mass as possible, so that it will be supported entirely by the aforementioned flexure joint and the weight of the rotor itself will not cause spurious outputs from the vibrating beam force transducers.
Some of the aforementioned difficulties have been met by the development of electrostatic rate gyroscopes in which a hollow spherical rotor is electrostatically suspended in the gyroscope housing and an electrostatic capture system is utilized to null the rotor with respect to the housing. In this arrangement, however, if the spherical rotor is made of relatively thin material to keep the rotor mass as low as possible, the rotor will tend to deform at high spin speeds because of the action of the high centrifugal forces involved. Since the accuracy of the spherical rotor type of electrostatic gyroscope depends to a large extent upon the sphericity of the rotor, the spin speeds of these gyroscopes are therefore kept within reasonble limits to prevent this centrifugal deformation. The aforementioned limitation on spin speeds limits the ratio of angular momentum to weight and therefore increases the weight which must be supported by the flexure joint. Accordingly, the limitation on spin speeds with the attendant limitation on weight reduction will produce a structure which is limited in its ability to be free from errors produced by vibration, shock and acceleration. Furthermore, for maximum operating efficiency, the electrostatic suspension system for the spherical rotor type of electrostatic gyroscope must be highly accurate in operation. The suspension electrodes must be so designed and located that spurious drag torques are not applied to the spherical rotor which would produce spurious outputs and gyroscope inaccuracy.